1. Field of the Invention
The present invention relates to a method for the orientation of a spindle of a numerically controlled and rapidly rotating spindle, by which the spindle is brought from an initial rotational speed into a predetermined position of rest in that in a first phase the spindle is braked at a first braking rate function to a threshold rotational speed and in a second phase the defined position of rest is controlled by position regulation. The present invention further relates to a numerical control device of a machine tool embodied for the method of the present invention.
2. Discussion of Related Art
Such a method makes it possible to bring a rapidly rotating spindle, for example that of a machine tool, to a stop in a defined position in a very short time, for example for the purpose of changing a tool on the spindle.
Spindles operating at increasingly higher numbers of revolution are used in modern machine tools. Cutter heads are used for so-called high speed cutting which are operated at 60,000 revolutions per minute, for example. In this way the processing speeds can be increased in comparison with slower tool spindles, and the processing times can be reduced. Because of this the productivity of a machine tool is clearly increased.
In the course of processing it is necessary time and again to brake the spindles from their high rotational speed and to bring them to a stop in a defined angular position or location. This can be necessary in case of a tool change, for example, if a tool can only be used in a defined orientation with respect to the spindle.
In a numerically controlled machine tool the movement of the spindle shaft is monitored by a numerical control device. By a cascading regulating structure consisting of a position controller, a rotational speed controller and current or torque regulators it is possible, at least at low numbers of revolutions, to control the spindle very accurately by presetting the respectively desired angular position in the position controller as the nominal position value. However, at very high numbers of revolutions a position regulation can no longer take place for reasons which will be explained in detail farther down below. It is therefore customary to bring rapidly rotating spindles to the desired rotational speed by a rotational speed controller. For this purpose the position controller is deactivated and a nominal rotational speed value is provided directly to the rotational speed controller.
If processing of a workpiece requires the frequent stopping of the spindle in a defined position, the time required for this becomes an important factor in the throughput at such a machine tool. Therefore methods are already known which permit a spindle orientation as fast as possible, even at high numbers of revolutions. All these methods have in common that initially the high rotational speed is reduced under the control of a rotational speed controller until a rotational speed has been reached which can also be controlled by a position controller.
Thus, EP 0 580 866 B1 describes such a method for the orientation of a spindle. In accordance with this method, following the generation of a spindle orientation command, first the rotational speed is reduced to a threshold rotational speed N1 under the control of the number of revolution regulator, in that this threshold rotational speed is preset in the rotational speed controller. Only after this threshold rotational speed has been reached are parameters calculated, which subsequently under position regulation permit the desired position of rest to be approached on a time-optimized track.
The disadvantage of methods of this type lies in that, after reaching the threshold rotational speed at which the switching over from the rotational speed regulation to the position regulation is to be made, the spindle first must be operated for some time at the threshold rotational speed in order to perform the required calculations for switching over into position regulation. This means a considerable loss of time for the spindle orientation, since during this time no further braking of the spindle occurs. In connection with the known methods it is furthermore customary to switch over from maximum deceleration to a continuous rotational speed when reaching the threshold rotational speed. Because of this the first derivative with respect to time of the acceleration, the jerk, becomes very large, from which a high stress of the mechanical parts results.